This disclosure relates to electrical circuits.
Radio frequency (RF) power amplifiers for conventional wireless communication applications can be subjected to elevated voltages. Conventional RF power amplifiers are typically constructed using, e.g., gallium arsenide (GaAs) heterojunction bipolar transistors (HBTs) or silicon germanium (SiGe) bipolar transistors, which can break down under such elevated voltages. For example, a sub-micron (e.g., 0.35 μm) SiGe bipolar transistor has a base-collector breakdown voltage of approximately 5-8 volts. The elevated voltages can occur due to output load mismatch, and the like. Output load mismatch can occur, for example, when an antenna that is being driven by an RF power amplifier comes into contact with a foreign object or when a transmitter switch is open. Under mismatched conditions, the voltage at the collector of an output transistor can exceed the transistor's base-collector breakdown voltage.
FIG. 1 shows a graph of voltage vs. time for a collector of an output transistor of a wireless device. During the time shown a continuous mismatch condition was present (e.g., the antenna was in contact with a foreign object.) The device used was powered by a 3 volt supply, however, as shown in the graph the peak voltages at the collector often exceeds twice the power supply voltage.
A conventional peak detection circuit can be used to avoid excessive collector voltages. A conventional peak detection circuit pulls the base node of the output transistor to ground (i.e., turns the output transistor off) upon detection of a collector voltage peak 100 that is greater than, e.g., 5 volts. As the output transistor turns off, the collector voltage falls to zero. The output transistor turns back on at point 102 as the base node of the output transistor approaches the base-emitter threshold (e.g., 0.7 volts). Due to the continuous mismatch, a second collector voltage peak 104 occurs shortly thereafter, and the conventional peak detection circuit turns off the output transistor. This cycle repeats as long as the mismatch remains, as represented by subsequent collector voltage peaks 106, 108.
As shown in FIG. 1, each of collector voltage peaks 100, 104, 106, 108 contain multiple collector voltage swings above the 5 volt threshold of the conventional peak detection circuit. When the output transistor turns on (e.g., at point 102), the amplitude of the collector voltage rises, having an envelope slew rate greater than can be tracked by a conventional peak detection circuit—i.e., the conventional peak detection circuit cannot respond in time to prevent collector voltage swings above the 5 volt threshold. In the example of FIG. 1, the conventional peak detection circuit is unable to detect a collector voltage swing above the 5 volt threshold until approximately 4-5 nanoseconds after the collector voltage first exceeded the 5 volt threshold. The periodic, multiple collector voltage swings above the 5 volt threshold can lead to breakdown of the output transistor, and failure of the RF power amplifier.